It is disclosed in Japanese Patent Publication Nos. 9,309/65, 14,039/70 and 29,830/76 that a phenol compound is used as an electron accepting compound for pressure-sensitive recording materials and heat-sensitive recording materials. However, the phenol compound is not sufficient for the electron accepting compound, because color density of images is not high and color fasteness of images to light, humidity, etc. is not sufficient where it is used in combination with an electron donating colorless dye.
A well-known sulfon compound causes fog which means color formation without pressure or heat. For example, bisphenol S provides more fog by humidity because it is more water-soluble. A hydroxydiphenylsulfon compound as disclosed in Japanese Patent Publication (unexamined) No. 210,886/82 is not satisfactory electron accepting compound because it provides more fog due to water-solubility.